1. Field of the Invention
This invention relates to oxygen scavenging particles and manufacturing methods thereof having utility in packaging, particularly suitable for incorporation into film-forming polymers, preferably aromatic polyester resins and the wall of a container made from the aromatic polyester containing the scavenging particle.
2. Description of Related Art
Products sensitive to oxygen, particularly foods, beverages and medicines, deteriorate or spoil in the presence of oxygen. One approach to reducing these difficulties is to package such products in a container comprising at least one layer of a so-called “passive” gas barrier film that acts as a physical barrier and reduces or eliminates the transmission of oxygen through the container wall but does not react with oxygen.
Another approach to achieving or maintaining a low oxygen environment inside a package is to use a packet containing a rapid oxygen absorbent material. The packet, also referred to as a pouch or sachet, is placed in the interior of the package along with the product. The oxygen absorbent material in the sachet protects the packaged product by reacting with the oxygen before the oxygen reacts with the packaged product.
Although oxygen absorbents or scavenger materials used in packets react chemically with the oxygen in the package, they do not prevent external oxygen from penetrating into the package. Therefore, it is common for packaging using such packets to include additional protection such as wrappings of passive barrier films of the type described above. This adds to product costs.
In view of the packet or sachet's disadvantages and limitations, it has been proposed to incorporate an “active” oxygen absorbent, i.e. one that reacts with oxygen, directly into the walls of a packaging article. Because such a packaging article is formulated to include a material that reacts with the oxygen permeating through its walls, the package is said to provide an “active-barrier” as distinguished from a passive barrier that merely blocks the transmission of oxygen but does not react with it. Active-barrier packaging is an attractive way to protect oxygen-sensitive products because it not only prevents oxygen from reaching the product from the outside, it can also absorb oxygen present within a container wall, and absorb the oxygen introduced during the filling of the container.
One approach for obtaining active-barrier packaging is to incorporate a mixture of an oxidizable metal (e.g., iron) and an activating component which promotes the reaction of the metal with oxygen, often in the presence of water, into a suitable film-forming polymer. Examples of activating components are electrolytes (e.g., sodium chloride), acidifying components, electrolytic acidifying components, or protic solvent hydrolysable halogen compounds like Lewis acids (e.g. aluminum chloride). In the case of nano-metals, little or no activating component may be needed due their inherent pyrophoricity.
The scavenger containing film forming polymer is then melt processed into a monolayer or multilayer article such as a preform, bottle, sheet or film that eventually forms the resulting oxygen scavenger-containing wall or walls of the rigid or flexible container or other packaging article. It will be understood that a film-forming polymer is one that is capable of being made into a film or sheet. The present invention is not, however, limited to films and sheets. Examples of such film forming polymers are polyamides, polyethylenes, polypropylenes, and polyesters.
The container of the present invention also includes bottle walls, trays, container bases, or lids. It should be appreciated that references to the container sidewall and container wall also refer to the lid, bottom and top sides of the container, and a film that may be wrapped around the product such as meat wraps.
One difficulty with scavenger systems incorporating an oxidizable metal or metal compound and an electrolyte into a thermoplastic layer is the inefficiency of the oxidation reaction. High loading of scavenger compositions and relatively large amounts of electrolyte are often used to obtain sufficient oxygen absorption scavenging rate and capacity in active-barrier packaging.
According to U.S. Pat. No. 5,744,056, oxygen-scavenging compositions that exhibit improved oxygen-absorption efficiency relative to systems such as iron and the electrolyte sodium chloride are obtainable by including a non-electrolytic, acidifying component in the composition. In the presence of moisture, the combination of the electrolyte and the acidifying component promotes the reactivity of metal with oxygen to a greater extent than does either alone. However, the acidifying component when used alone does not exhibit sufficient oxygen-scavenging properties.
A particularly preferred oxygen-scavenging composition according to the U.S. Pat. No. 5,744,013 comprises iron powder, sodium chloride and sodium acid pyrophosphate, in amounts from about 10 to 150 parts by weight of sodium chloride plus sodium acid pyrophosphate per hundred parts by weight iron.
These conventional scavenging compositions are created by dry blending the ingredients or depositing the acidifying agents and salts onto the metal particle out of an aqueous liquid or slurry and then regrinding the composition, thus creating more particles.
U.S. Pat. No. 5,744,056 teaches that the degree of mixing of the oxidizable metal, electrolyte and acidifying components and, if used, optional binder component has been found to affect oxygen absorption performance of the oxygen-scavenging compositions, with better mixing leading to better performance. Mixing effects are most noticeable at low electrolyte plus acidifying components to oxidizable metal component ratios and at very low and very high acidifying component to electrolyte component ratios. Below about 10 parts by weight electrolyte plus acidifying components per hundred parts by weight metal component, or when the weight ratio of either the electrolyte or acidifying component to the other is less than about 10:90, the oxygen scavenger components are preferably mixed by aqueous slurry mixing followed by oven drying and grinding into fine particles. Below these ratios, mixing by techniques suitable at higher ratios, such as by high-intensity powder mixing, as in a Henschel mixer or a Waring powder blender, or by lower intensity mixing techniques, as in a container on a roller or tumbler, may lead to variability in oxygen uptake, particularly when the compositions are incorporated into thermoplastic resins and used in melt processing operations. Other things being equal, U.S. Pat. No. 5,744,056 claims that oxygen-scavenging compositions prepared by slurry mixing have the highest oxygen absorption efficiency or performance, followed in order by compositions prepared using high intensity solids mixers and roller/tumbler mixing techniques.
U.S. Pat. No. 4,127,503 teaches the dissolution of an electrolyte in water, contacting the solution with the oxidizable component (e.g. iron) and then removing the water from the composition. While this technique is suitable for salts which dissolve into water, it is not suitable for salts which hydrolyze in the presence of a protic solvent, such as water. Aluminum chloride for instance, will hydrolyze in the presence of water to hydrochloric acid and aluminum hydroxide.
Incorporation of dry blends into the wall of clear containers is difficult because of the haze and colour brought on by the number of discrete particles. United States Patent Applications 20030027912, 20030040564, and 20030108702, teach that using larger oxidizable particles minimizes the number of particles and improves the haze and colour of the transparent wall of the container. As taught by these patent applications, the goal of oxygen scavenging compositions should be to have as few particles as possible.
Another deficiency of using dry blended or ground conventional oxidizable metal compositions is the growth of the particle as it oxidizes. It has been observed that as the particle oxidizes, the oxidized material blooms away from the particle making the particle appear larger over time and the colour shifts towards the colour of the oxidized metal. In the case of iron, the colour of the container wall shifts to yellow and yellow orange (rust).
Beverage or food containers presenting the above blooms are commercially unacceptable because the consumer incorrectly attributes the colour to deterioration of the product inside the container.
European Patent Application EP-1 506 718 titled Oxygen Scavenging Compositions and the Application thereof in Packaging Containers filed Aug. 14, 2003 and World Patent Application WO-2005/016 762 titled “Oxygen-scavenging compositions and the application thereof in packaging and containers” submitted on Aug. 11, 2004 teaches that certain protic solvent hydrolysable activating components can be placed onto the oxidizable component by dissolving the activating component into an essentially moisture free organic solution, contacting the solution with the oxidizable metal then removing the solvent.
While the deposition of compounds from a liquid phase achieves the desired intimacy of contact for a unitary particle, liquid phase deposition presents several problems. First, there are the impurities of the solvent or reaction products of the salt with the solvent, often called adducts. These may or may not be bound into the composition. Second, the liquid phase deposition requires a dissolution step and a solvent removal step.
A third drawback of liquid deposition is that the penetration of the liquid into the pores of many metal particles may be inhibited by the surface tension of the liquid.
Yet, another deficiency of liquid deposition is the instability of the liquid deposited composition during further heat processing of the polymer containing the liquid deposited oxygen scavenger. In the case of polyesters, it is advantageous to place the scavenger into the low molecular material and then subject the polymer to solid state processing often at 225° C. for 16-20 hours. As discussed later, bottles and preforms made from polymer containing a liquid deposited oxygen scavenger were unacceptably yellowed relative to the particles made from this invention.
Japanese Patent Application 09-237232 also describes depositing the activating component from an aqueous or organic solution and placing it into the wall of a container. The container wall of Japanese Publication Number 11-080555 (Patent Application 09-237232) is a laminate of metal foil and plastic containing the oxygen scavenger lying between the foil and the package contents. The container is thus non-transparent and any advantage of reducing the number of scavenging particles is not appreciated.
Reacting the outer surface of the iron particle with a compound in a vapour stream is another way to achieve intimate contact. Japanese Publication Number 11-302706 (Application Number 10-131379), titled “Iron Powder For Reactive Material and Its Production” teaches placing an enveloping layer containing 0.1-2% of the weight of chlorine in the iron powder which the enveloping layer which becomes a front face of [sic] ferric chloride by contacting hot chlorine or hydrogen chloride gas to iron powder. This way the ferric chloride is made to form in the front face of said iron powder.
Although this vapour phase-solid phase reaction creates intimacy of contact, it limits one to the reaction products of iron and various gasses. Because this particular Japanese disclosure requires that the oxidizing agent be a reaction product of iron, the practitioner is limited by the kinetics of the iron based salts and iron. Dissimilar metals such as aluminum chloride and iron are not available with this technique.
U.S. Pat. No. 6,899,822 teaches the use of an acidifying electrolyte such as sodium bisulfate in the presence of sodium chloride and iron. However, none of the examples teach depositing the materials onto the iron.